Ferrules are employed as a portion of the structural components of connectors used to join quartz group optical fibers. Optical connectors are designed to accurately align the positions of and connect the cores that are in the center of the optical fibers by passing optical fibers that are around 0.125 mm thick through the ferrules and securing them.
Among the ferrules that are currently in use, there are stainless steel, zirconia and plastic ones, but those made of zirconia make up the largest proportion. In producing ferrules made of zirconia, first, zirconia powder and a resin are mixed together; this is molded into a cylindrical form by means of injection molding, extrusion molding, and the like, the molded item is heated at a temperature of around 500° C.; and the resin component is decomposed and eliminated. The product is then fired at a high temperature of around 1,200° C. After firing, the product is cut to a specified length, a wire shaped diamond grinder is passed through the hole in the center of the cylinder, and precise dimensions related the inside diameter are obtained. This grinding work is carried out by hand and accuracy is demanded. In addition, boring is performed and the ferrule is finished with the polishing of the end surfaces. Furthermore, in order to improve the degree of coaxiality of the inside and the outside diameters, the processing is done using a wire centerless machine. Even though the several processes are carried out in this way, the actual state of affairs is that irregularities are produced in the inner and outside diameters, and the coaxiality and the ferrules are individually inspected and separated according to their dimensions.
High-cost injection molding equipment, extrusion molding equipment, and molds are required for the production of zirconia ferrules. In addition, such problems have been pointed out as the fact that the life of the molding equipment and molds is short, the energy cost is high due to the processing of the zirconia-resin molded items at the high temperatures of 500 to 1,200° C., the center hole must be ground with a wire shaped diamond grinder in order to produce the precise hole dimensions, and the productivity is low because the grinding work is done manually by highly skilled workers.
The present invention is one in which metal ferrules are obtained by electroforming instead of producing ferrules made of zirconia. The manufacture of fine-hole pipe by means of electroforming is already known. For example, in Japanese Patent Application Laid-open No. 11-193485, a method for manufacturing tubes having fine holes is disclosed in which a metal film is formed on the surface of a core material and the core material is removed leaving behind the metal film that has been formed. In addition, in Japanese Patent Application Laid-open No. 56-90995 and Japanese Patent Application Laid-open No. 4-311589, a method for manufacturing fine diameter pipe is disclosed in which metal is electroformed and plated on the outer surface of a core wire that can be dissolved by a chemical and, after it is cut to a specified dimension, the core wire is dissolved with the chemical and removed.
In other words, it is a method for manufacturing pipe having a fine diameter in which, after a wire core is prepared and the core wire is electroconductively processed, metal is electrodeposited around the periphery of the core wire by means of an electroforming operation and, following that, the core wire is removed with an appropriate method. Basically, by using this method, it is possible to produce ferrules for connecting optical fiber.
The diameter of the optical fiber is, in conformance with the standards, 0.125 mm and, consequently, the inside diameter of the ferrule should be around 0.126 mm. The length of the ferrule itself is 12 mm or less and the outside diameter is around 2.5 mm. There are various problems related to the production of this kind of small and fine diameter pipe by electroforming. In particular, there are the problems regarding productivity and dimensional accuracy.
In order to increase the productivity, a method is advocated in which a plurality of comparatively long core wires are used with the electroforming done at the same time and, after it is performed this way, they are cut to a specified length. When long core wires are used and, moreover, a plurality of core wires are electroformed at the same time, the problem of uniformity is especially great. Since a high degree of circularity and coaxiality is demanded of the ferrules, the thickness of the electrodeposition layer must be uniform in the longitudinal direction for each of the core wires and, at the same time, the thicknesses of the electrodeposition layers of the multiple number of core wires must be uniform.
The uniformity problems are thus related to the circularity of the ferrules, that is, the way in which the inside and outside diameters of the ferrules can be made uniform, and the coaxiality thereof, that is, in other words, the way in which the axial centers of the and the outside diameter are aligned.
For example, when a stainless steel round wire having a circular cross-section with a diameter of 0.125 mm is used as the core wire and electroforming is carried out with this connected to the power source as the cathode, as is shown in FIG. 3(b), an electroforming with a nonuniform amount of electrodeposition is obtained in which the diameter of the upper part is large and the diameter of the lower part is small. This is because the current density of the stainless steel core wire is high for the portion near the power source, but the current density becomes lower the further the area is from the power source due to the electrical resistance. Since the electrodeposition amount is proportional to the current density, the amount of electrodeposited metal becomes greater the closer the area is to the power source where the current density is high, and the amount of electrodeposited metal becomes less the further the area is from the power source. As a result, the diameter becomes larger the closer the area is to the power source, where the current density is high, and the diameter becomes smaller the further the area is from the power source.
The phenomenon of the nonuniformity of the electroforming diameter, where the outside diameter of one end of the casting becomes large and the outside diameter becomes smaller as the other end is approached, is a large problem in those cases where items small in size, such as ferrules used for optical fiber, are produced. In particular, it is a problem when a method is employed as a means to increase the ferrule productivity in which a long length of pipe is produced, and that is cut into specified lengths.
In addition, in those cases where metal balls are used as the anode, the metal balls are inserted into an electroconductive netlike tube or the like. When the core wire is electroformed, the shape of the electroforming is largely dependant upon the shape of the tube into which the metal balls have been inserted. For example, in those cases where the shape of the tube is one that bulges outward, the outside diameter of the casting becomes larger in the area that corresponds to that position and, in those cases where the outside of the tube is indented inward, the outside diameter of the corresponding portion of the casting becomes smaller.
In other words, as is shown in FIG. 13(a), the metal lumps 64 of nickel are accommodated in the metal tube 61 made of titanium. The metal tube 61 becomes narrow toward the inside in the middle area. When this metal tube 61 is employed and electroforming is carried out, the resulting casting becomes unacceptably narrow in the area that corresponds to the portion in which the metal tube is indented inward, as shown in FIG. 13(b).
Another drawback is that the factors that cause the dimensions of the electroforming to become nonuniform are dependant upon the degree of extensibility of the core wire. In other words, when the tension that is applied to the core wire is great, the core wire is stretched in the longitudinal direction and differences are produced in the diameter of the core wire. Together with this, differences are also produced in the outside diameter of the resulting casting, making it impossible to obtain a ferrule that is uniform and has a high degree of dimensional accuracy. In addition, when the tension is low, slackness is produced in the core wire, and this factor impairs the uniformity of the resulting casting in the same manner.
The present invention provides a method and an apparatus with which ferrules having a small inside diameter, uniform inside and outside diameters, and a high degree of coaxiality are produced with good efficiency and high productivity, particularly when small ferrules having an inside diameter of around 0.126 mm are produced by electroforming.